I can’t stress the importance of the master password in
Firefox if you are one of the users who is letting Firefox save passwords for you so that you do not need to enter the password and username again when visiting the site at a later time. While this feature is surely comfortable and takes away the need to memorize usernames, passwords and corresponding sites plus the typing that has to be done when logging into a site it is a security risk.
Even more so if you have not set the master password. Anyone with access to your computer is able to go into Tools > Options and click on
Show Passwords under the Security tab. What this does is that it lists all sites and usernames that Firefox has stored in its
database.
Pressing the button ‘Show passwords’ in that new menu displays all corresponding passwords for all sites. Gmail, Blogs, Myspace, nothing is really safe. By taking a look at your passwords someone could also analyze patterns. If you were using the same passwords on all sites it is fairly easy to assume that your
pop3 email account would most likely use the same password.
So, set your master password to be on the safe side here.
Make sure you set a master password in Firefox
Make sure you set a master password in Firefox 
Also, saving passwords with firefox is the fastest way to forget those passwords.
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